Structures and vessels that are supposed to stay moored at the same location for a long time, such as months or years, needs to be moored with a prescribed tensioning. The traditional method is to use chain tensioning equipment on deck for pull-in and tensioning. Normally this equipment is used during the installation period and left on the vessel for several years without being used or maintained. When the mooring lines slacken sufficiently over time, due to wear of chains, shifting of anchors, creep in fiber mooring lines etc, the mooring lines need to be re-tensioned, repositioned (typically move the chain one or two chain links in order to engage the chain stopper on a new chain link) or replaced. In this period between the installation and the next operation, the equipment has normally seized or corroded and requires refurbishment.
Another issue is the weight and deck space. Deck space is often limited and weight on deck an important factor for the stability of the vessel.
The aim of this patent application is to provide arrangements and methods where there is little or no requirement for equipment on the deck of the vessel and still be able to perform the installation, tensioning, re-tensioning, re-positioning and replacement operations.
US 2014/0216323 describes a mooring arrangement and a method of installing the mooring arrangement. An anchor chain is attached to the seabed by an anchor. A submerged chain stopper is attached to an upper end of the anchor chain. A pull chain or installation chain extends from the structure to be anchored through the chain stopper. The lower end of the installation chain is pulled upwards by the use of a winch on the support vessel until the installation chain passes through the chain stopper, a prescribed tension is achieved and the chain stopper engages on the permanent part of the mooring line, here described as the work chain.
US 2002/0189522 describes a similar arrangement and method as above.
WO 03/013950 also describes a similar mooring arrangement and method.
The main difference between the above mentioned patents are that the US 2014/0216323 A1 may use a wire from a winch on the anchor handler towards the platform. By doing this it is possible to tension the mooring line with a force based on the winch capacity. The actual tension force will depend on factors such as angle of the mooring chain with respect to the platform and the support vessel. If the mooring line is close to vertical, the effect of this method is close to zero.
In these references the arrangement and method are relied on pulling the installation chain of the mooring chain vertically or close to vertically upwards to an installation vessel in order to tension the chain. The benefit of both these systems is that there is no chain and chain handling equipment on the deck of the FPSO. However, the main disadvantages are that all mooring line tensioning requires an installation vessel and fine tuning of each mooring line may be difficult. Additionally, the installation requires an additional length of installation chain.
It is expensive to use an installation vessel every time a re-tensioning of the mooring is required.
The vertical, or close to vertical, pulling of the chain will also pull the submerged chain tensioner upwards, so that an obtuse angle is created between the chain below the chain tensioner and the chain above the chain tensioner extending between the anchor at the seabed and the moored structure, as can readily be seen in the figures of the two references. The size of this angle will depend on several factors, such as pulling tension, weight of the chain, friction in reversing pulley of the chain tensioner, sea currents etc. Due to the angle, the length of the chain between the anchor and the floating structure is somewhat greater than the linear distance between the anchor and the floating structure.
Especially if the anchor is at a great distance from the floating structure or the water depth is small, the mooring line can extend at a very shallow angle from the seabed. In such cases the pull to tighten the mooring can be almost at right angle to the mooring line. This will result in the angle between the two parts of the mooring line, below and above the chain tensioner, becoming smaller and thus the length of the mooring line during tensioning will become far greater than the direct distance between the anchor and the floating structure.
When the mooring has been tightened to the prescribed tension, the upper end of the installation chain will be released. Hence, the chain tensioner will sink until it finds itself approximately on the straight line between the anchor and the floating structure. This inevitably leads to a slackening of the mooring. Hence, the mooring must be tightened somewhat beyond the required tension to account for this slackening.
However, it is difficult to predict how much overtightening is required to achieve the correct tension.
There are several other disadvantages as well, such as:                The weight of the chain stopper arrangement may be a problem for the mooring line and the mooring characteristics.        After the chain installation and tensioning is finished, the excess mooring chain has to be cut off to reduce the additional weight midwater. This will typically require a ROV operated subsea chain cutter unit.        If you want to pay out the mooring chain, the chain stopper will have to be operated with an ROV. The chain tensioning arrangement will typically be located 50-100 meters below the surface.        
Objectives of the Present Invention
The present invention has as a first main objective to avoid additional weight midwater on the mooring line from the chain stopper structure and the excess mooring chain. A wire sheave positioned mid-water has considerable less weight than a chain stopper pulley and excess mooring chain.
The mid-water sheave may, in an alternative embodiment, be a temporary sheave arrangement, which is removed after the installation has been completed.
The present invention has as a second main objective to avoid the problem of having to account for a certain amount of overtightening when installing a mooring or re-tensioning a mooring.
A further objective of the present invention is to provide an arrangement and method for installation, tensioning and replacement of mooring lines where there is no requirement for chain handling on the deck of the floating structure.
Tensioning can be performed from an installation vessel and in particular smaller vessels due to multiplication of the tensioning force from the pulley arrangement. The multiplication factor is close to 3 depending on actual angle and friction.
Additionally, a slightly longer work chain can be used instead of an installation chain. An installation wire may act as the main part of the installation chain.
Yet another advantage of the invention is that the chain tensioning may be operated from the FPSO by a wire, rope or hydraulic cylinder.
Another advantage of the present invention is that the weight of the excess top chain is carried mainly by the hull bracket.